Use separate actions to select/deselect investments

This is consistent to what we usually do. Also, we're applying the same
criteria mentioned in commit 72704d776:

> We're also making these actions idempotent, so sending many requests
> to the same action will get the same result, which wasn't the case
> with the `toggle` action. Although it's a low probability case, the
> `toggle` action could result in [selecting an investment] when trying
> to [deselect] it if someone else has [deselected it] it between the
> time the page loaded and the time the admin clicked on the
> "[Selected]" button.

app/controllers/admin/budget_investments_controller.rb

@@ -9,7 +9,7 @@ class Admin::BudgetInvestmentsController < Admin::BaseController
   has_filters %w[all], only: :index
 
   before_action :load_budget
-  before_action :load_investment, only: [:show, :edit, :update, :toggle_selection]
+  before_action :load_investment, except: [:index]
   before_action :load_ballot, only: [:show, :index]
   before_action :parse_valuation_filters
   before_action :load_investments, only: :index
@@ -60,10 +60,18 @@ class Admin::BudgetInvestmentsController < Admin::BaseController
     end
   end
 
-  def toggle_selection
-    authorize! :toggle_selection, @investment
-    @investment.toggle :selected
-    @investment.save!
+  def select
+    authorize! :select, @investment
+    @investment.update!(selected: true)
+
+    render :toggle_selection
+  end
+
+  def deselect
+    authorize! :deselect, @investment
+    @investment.update!(selected: false)
+
+    render :toggle_selection
   end
 
   private